This invention relates to running braces and in particular to energy-efficient running braces.
This invention augments the effective spring constant of the leg by adding a resilient brace which supports the runner's weight in parallel with the leg. Furthermore, it provides for a body brace to function as an exoskeleton to protect from impact shock to body organs and body connective parts such as the joints of the knees, back, and neck.
The act of running involves vertical motion of a runner's center of gravity. The lifting of the runner's weight requires muscle work. When the runner's foot impacts the ground, both the kinetic energy and the momentum associated with the vertical motion must be absorbed. Approximately 45% of the vertical kinetic energy is stored in the resilient parts of the leg and foot, with the remainder lost to the ground and the leg. The lost energy must be replaced by muscle work. This invention is intended to minimize lost energy or, equivalently, to maximize the energy efficiency of running.
Scientific inquiry of running includes the study of the efficiency of running as a function of various parameters. Researchers have been attempting to discover optimized running parameters for greater energy efficiency and to fewer injuries. Dr. Thomas A. McMahon of Harvard University discussed how the resiliency of tracks can be tuned to improve performance and safety in his article "Mechanics of Locomotion," T. McMahon, 3 Int. J. Of Robotics Research 4 (1984). One of his results is that running times improve by two or three percent and injuries are reduced by a factor of two when the effective spring constant of the track is approximately two times that of a runner's leg. Prior art running shoes cannot achieve improved energy efficiency equivalent to that achieved with tuned tracks, because impact is on the heel whereas take-off is from the toe. Prior art running shoes do not have a means for transmitting the impact energy from the heel to the toe.
Furthermore, the effective spring constant of a leg must be large to achieve high performance or speed. A drawback of tuned tracks is that the effective stiffness of the leg/track system is smaller than that of the leg itself, since the track acts as a spring in series with the spring representing the leg, and the spring constants of springs acting in series add reciprocally. The present invention solves that problem by having the braces act in parallel with the legs, in which case the spring constants add linearly.
Two important concepts are compliance and resilience. Compliance refers to the property of the sole to give or compress upon foot impact; resilience refers to the property of the sole to return to its original shape. This can be made clearer by referring to a spring model with damping. The term damping includes all friction losses. A spring system may be very compliant by virtue of having considerable damping, but then it is not energy efficient. Prior art running shoes have this drawback. The term resilience as used herein means that damping is minimized, so energy efficiency is maximized. In summary, compliance describes a system where impact energy dissipated, whereas resilience refers to a system where energy loss is conserved.
An example of an invention that provides compliance in a shoe is described in U.S. Pat. No. 4,446,634. This shoe has liquid-filled bladders under the heel and sole, and controls the heel compliance with an adjustable valve in between. Since provision is made for energy storage and release, however, this shoe would not be energy efficient.
U.S. Pat. Nos. 4,237,625 and 4,358,902 disclose energy-efficient shoes. These have liquid-filled bladders below the heel and ball of the foot and resilient material below these bladders. However, there is no provision to transmit the energy of heel impact to the front of the foot, to store it, and to release it during thrust. The only energy that might be returned during thrust is that stored in the resilient material below the front of the foot, and this would be a small portion of the impact energy. Accordingly, this shoe would not be very energy-efficient.
U.S. Pat. No. 4,451,994 discloses a shoe having a resilient mid-sole. This shoe cannot capture the heel-impact energy, nor can it give back much of the "midsole-impact" energy during thrust. U.S. Pat. No. 4,030,213 discloses a shoe with springs throughout the sole. This shoe may be compliant, but it would not be energy efficient, since there is no provision for transferring heel impact energy to the front of the shoe.
Leg-brace devices in prior art are called walking irons. U.S. Pat. No. 2,206,234, granted in 1940 and entitled an "Invalid Walking Aid Apparatus," has a brace which extends from the foot to the upper leg and it has a spring to cushion the impact of foot strike. However, contact with the ground is made via a pair of feet each of which is similar to the foot of a pogo stick. This device is appropriate for hobbling, whereas the present invention is intended for running because the foot strike involves the entire foot, thereby making possible better balance and stability. Another difference is that the cushion springs in the just-mentioned invention act in series with the effective springs of the legs, whereas the brace springs in this invention act in parallel, which results in a higher effective spring constant and greatly enhanced performance.
A important consideration is the protection of the connective parts of the upper body, mainly the back and neck, from impact shock transmitted through the leg brace, to the pelvis, and up the back. When the effective stiffness of the leg/brace system is high enough, the optional back and neck braces, and the accompanying shoulder and chin harnesses, are needed to act as an exoskeleton for the back and neck.